Make ErrorType DeclRefExprs for ErrorType Decls

This prevents one invalid declaration from causing cascades of subsequent errors.

Fixes:
* test/NameBinding/scope_map_lookup.swift
* test/decl/var/variables.swift

Author: beccadax

lib/Sema/CSGen.cpp

@@ -1352,8 +1352,10 @@ namespace {
       // We may, alternatively, want to use a type variable in that case,
       // and possibly infer the type of the variable that way.
       CS.getTypeChecker().validateDecl(E->getDecl());
-      if (E->getDecl()->isInvalid())
+      if (E->getDecl()->isInvalid()) {
+        E->setType(E->getDecl()->getInterfaceType());
         return nullptr;
+      }
 
       auto locator = CS.getConstraintLocator(E);